Outdated instructions to update index

[orontee]

According to https://epsg.org/download-dataset.html:

Download of the full dataset in GML is no longer supported. GML documents for individual entities may be accessed through EPSG Dataset / Text Search.

Thus the "Create your own index from EPSG export" from the README file should be updated and an alternative provided to build an up-to-date index.

[jtcooper10]

The live site appears to be using EPSG v10.076, which is much newer than the pre-built index of EPSG v9.8 in the repository.

In addition, there are some entries that don't seem to actually be present in the EPSG dataset. For example, EPSG:9377 is present in both the EPSG dataset v10.076 and in epsg.io. ESRI:103599, however, is present in epsg.io but I can't find it in the EPSG registry (likely due to it being ESRI data, or maybe I'm mistaken).

Am I correct in my assumption that, for the live site, the pre-built index is not being used in production and that this index includes data from outside of the EPSG dataset? If so, how is it done? Can the readme be updated to reflect this? There are some coordinate systems (and revisions) that are available in epsg.io but aren't available in the prebuilt index and, as mentioned, the instructions for updating it don't work anymore, since it appears that the only dataset format supported is now deprecated.